Wavelength division multiplexed (WDM) optical communication systems (referred to as “WDM systems”) are systems in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber using an optical multiplexer circuit (referred to as a “multiplexer”). Such systems may include a transmitter circuit, such as a transmitter (Tx) photonic integrate circuit (PIC) having a transmitter component that includes a laser associated with each wavelength, a modulator that modulates the output of the laser, and a multiplexer that combines each of the modulated outputs (e.g., to form a combined output or WDM signal).
A WDM system may also include a receiver circuit having a receiver (Rx) PIC. The receiver PIC may include an optical demultiplexer circuit (referred to as a “demultiplexer”) that receives an optical signal and demultiplexes the optical signal into individual optical signals. Additionally, the receiver circuit may include receiver components that convert the individual optical signals into electrical signals, and output the data carried by those electrical signals. The receiver components may include an optical source, such as a local oscillator (e.g., a laser device), to provide an optical signal used to modify a frequency of the individual optical signals provided by the demultiplexer.
The Tx and Rx PICs, in an optical communication system, may support communications over a number of wavelength channels. For example, a pair of Tx/Rx PICs may support ten channels, each spaced by, for example, 200 GHz. The set of channels supported by the Tx and Rx PICs can be referred to as the channel grid for the PICs. Channel grids for Tx/Rx PICs may be aligned to standardized frequencies, such as those published by the Telecommunication Standardization Sector (ITU-T). The set of channels supported by the Tx and Rx PICs may be referred to as the ITU frequency grid for the Tx/Rx PICs.
An optical source (e.g., a laser component in a Tx PIC and/or a local oscillator component in an Rx PIC) sometimes produces a level of phase noise in an optical signal provided by the optical source. The phase noise often interferes in the modulation, transmission, and/or processing of an optical signal, particularly when a WDM system includes a compensation system (e.g., a digital filter) to compensate for signal dispersion (e.g., chromatic dispersion and/or some other type of dispersion). Lower frequency portions of the optical signal phase noise spectrum, in particular, sometimes cause substantial performance penalties in transmission of the optical signal (e.g., higher bit rate errors during processing of the optical signal).